The advancement of pulsed ion beam technology has been motivated by the potential to use such beams in applications requiring the bombardment by or deposition of a precisely controlled quantity of charged particles onto very small well defined target areas. Such applications include implantation of ions on semiconductor surfaces to produce components of an integrated circuit, sputtering from localized areas of a target to analyze the chemical nature of the surface using MS (mass spectroscopy), producing chemical changes in small localized areas such as are required in developing photo resist, and chemical milling to produce tiny well defined features on the target surface.
The ideal pulsed beam for these applications has a small cross section, is pure in terms of the absence of neutral particles and spread in the range of particle weight (elimination of isotopes and/or ion species having identical charge but different weight) and minimal range of energy (velocity) of the ions.
FIG. 1 shows a "column" 11 for generating a pulsed ion beam 13 according to the present state of the art. The beam 13 originates in source 15 and is formed by "beam forming" lens 17. Beam 13 then passes through "beam defining" aperture 19 which limits the outer envelop (divergence) of the beam 13. Beam 13 then passes through a crossover point 21 which is midway between two blanker electrodes 10 and through a "blanking" aperture 25. Objective lens 27 focuses beam 13 onto the target surface 29. A pulsed field applied between blanker plates 23 periodically deflects the beam 13 so that the beam is cutoff periodically by blanking aperture 25 thereby generating the pulsed beam.
The cross sectional area of the beam 13 (divergence of the beam 13 from the cross over location 21) is determined by the size of the "beam defining" aperture 19 and illustrated by the comparison of FIG. 1 showing a large beam defining aperture 19A to FIG. 2 showing a small beam defining aperture 19B. In columns of the present art, the user can select the size of the aperture (19A or 19B) corresponding to whether he requires a large spot size with a large current or a small spot size with a small current. According to the present art, the size of the blanking aperture 25 is fixed by the largest size of beam cross section that the user requires. However the use of a large blanking aperture places a limit on the minimum spot size that can be achieved by reducing the size of the beam defining aperture for reasons which are discussed in connection with FIGS. 3 and 4.
Although I do not wish to be bound by any theories that are presented in this specification, it is believed that the following discussion illustrates the mechanics of the motion of the beam during cutoff which results in an effective blurring of the beam spot at the target.
Motion of the beam during cutoff is illustrated in FIG. 3 and 4. (prior art).
As illustrated in FIG. 3, the deflection of the particle (defined as the deflection angle .alpha.) experienced by a particle depends upon the position of the particle between the electrodes in the direction of travel at the instant when the deflecting field is applied. Therefore, the beam does not experience a sharp cutoff when the blanking field is applied but rather a graded reduction of intensity and a divergence of the beam which is manifested as motion of beam due to cutoff.
FIG. 3 shows opposed blanking plates 10 and the position of a beam of particles 12 (three are shown) at the instant (t=0) that a voltage, V, is applied between the plates to deflect the beam so as to turn the beam off as the particles of the beam travel from left to right. The blanking plates have a thickness, D. Each particle is at distance "d" from the exit edge 14 of the blanker plates at the instant the blanking voltage is applied. Blanking aperture 25 that limits the greatest deflection angle of any particle of the beam. Then it can be understood by one having skill in the art that any particle (labelled as the n.sup.th particle) which is located a distance, d.sub.n, from the exit edge of the blanking plates at the instant the blanking voltage is applied will be diverted so that as it emerges from between the plates, will travel along a straight path given b y EQU y.sub.n =(Fd.sub.n x)/V.sup.2 +(Fd.sub.n.sup.2)/2V.sup.2
where:
x and y are coordinate axes, y being the "vertical" distance from the undetected beam and x is the "horizontal" distance from the edge of blanking plates, PA1 F is the force on the particle between the blanking plates equal to the deflection voltage, V, multiplied by the charge on the particle and divided by the separation of the plates; PA1 d.sub.n is the distance along the x axis of the "n.sup.th " particle from the exit edge of the deflector plates at the instant when the deflection voltage is turned on; PA1 D is the width of the deflection plates measured in the direction of the ubdeflected beam.
The effect of each particle following its unique path is that, after the cutoff voltage is applied, the "n.sup.th " particle will appear to pass through a location having coordinates, x.sub.n =D/2 (the center location between the edges of the plates) and y.sub.n (at a distance y.sub.n from the undetected beam) given where EQU y.sub.n =(Fd.sub.n D-Fd.sub.n.sup.2)/2V.sup.2
y.sub.n is a quadratic function of d.sub.n having a maximum at the center of the blanking plates, i.e., d.sub.n =D/2.
When the beam is turned back on, the paths of the particles again diverge from one another producing a motion of the beam which is in a direction opposite the direction occurring with cut off. FIG. 4 is a plot of y.sub.n vs. d.sub.n for both the turn off and turn on situations.
The effect of the divergence of the beam paths is to cause an apparent growth in the cross section of the beam at the midpoint of the blanker plates by an amount illustrated as 2y.sub.n in FIG. 4. thus giving a larger (blurred) spot at the target site. The problem becomes especially severe for pulse lengths of less than about 50 nsec. There are also excessive neutral particles which are undetected by the blanker and therefore contaminate the target surface when they reach the sample.
As discussed above, blurring of spot due to turning the beam off increases as the size of the blanking aperture is increased. However, selecting a size of the blanking aperture to reduce blur when using a small beam defining aperture results in a reduction of the maximum beam current that can be achieved when using a larger beam defining aperture. Although simultaneously selecting a small blanking aperture for use with a small beam defining aperture might be considered, the approach of replacing BOTH apertures is prohibitively inconvenient.
Advancement in the art has included the development of smaller and more efficient ion sources and improvement in the lenses used to form the beam.
The ion source of the present state of the art providing the smallest ion source is the liquid metal ion source in which the ion source is a needle to which an electrical potential is applied to develop a strong field at the tip of the needle. The strong field induces liquid metal wetting the needle to be drawn to the tip where the liquid metal forms an even sharper point. The lower limit of the size of the area emitting ions at the end of the needle is determined by charge interaction between the ions at the point. Areas having a breadth of about 500 .ANG. are achieved by this technique.
Elimination of neutral particles from the beam is achieved by the so-called "chicane" technique in which a beam is deflected by a field so that it passes around a "blocker" (shield). Neutral particles are not deflected by the blocker but separate from the beam and strike the shield, thereby removing them from the beam.
Other disclosures have appeared cataloging the advancements in FIB (focused ion beam) technology.
For example, U.S. Pat. No. 5,369,279 to Martin discloses a chromatically compensated paricle beam column featuring a needle type ion source, one or more round lenses and a plurality of interleaved quadrupole lenses which are intended to reduce chromatic aberration. Chromatic aberration is understood in FIB context as being a spread in the focal length of a lens due to a spread in the energy of the ions.
U.S. Pat. No. 5,194,739 to Sato et al discloses a liquid metal ion source of Ce ions forming a beam focused to a microspot.
U.S. Pat. No. 5,294,794 to Davies discloses a system for compensating for spread in flight time by a circuit that meaures actual flight time of a given mass of ion and then adjusts a drift field to accommodate to deviations of velocity.
U.S. Pat. No. 5,289,010 to Shobet discloses ion implantation having a target with pulses of high voltage applied to the target periodically to implant ions. The plasma is "purified" by passing the plasma through an ion ion plasma resonance system.
A critical review entitled "Focused Ion Beam Technology and Applications" by Meingallis published by the J. Vac. Sci. Technology B 5 (2), Mar/Apr 1987 is incorporated in this specification for further reference in the field.
None of the disclosures of these prior an addresses the problems associated with manual selection of the aperture as discussed above.